[Haskell-cafe] New type of ($) operator in GHC 8.0 is problematic

Rustom Mody rustompmody at gmail.com
Thu Feb 11 17:57:29 UTC 2016

Glad to hear a teacher-pov Richard!

On Thu, Feb 11, 2016 at 5:32 AM, Richard A. O'Keefe <ok at cs.otago.ac.nz>

> Just as a thought experiment: The FTP had a landslide support on the
>> libraries list.
>> How would it have fared on a Haskell-Edu list?
> Judging by the list of changes at
> https://wiki.haskell.org/Foldable_Traversable_In_Prelude
> the changes were mostly some extra classes (Monoid, Foldable,
> Traverseable) showing up
> in Prelude and a bunch of type changes to functions:
> ... [x] ...   changing to (Foldable t) => ... t x ...
> ... [x] ...   changing to (Traverseable t) => ... t x ...
> This is actually quite an interesting change.
> Using the same names *consistently* across a wide range of types
> makes programs easier to write and easier to read.
> From an educational point of view, you can't say
>   "We didn't need the Prelude to write `all` for us.
>    We could have written
>     all p (x:xs) = p x && all p xs
>     all _ [] = True
>   "
> any more because that has the wrong (old, list-specific) type.
> You *can* say
>    "We could have written
>     all = foldr True (&&)
>   "
> So you lose a lesson that comes somewhere near the beginning,
> when you are still trying to get across the idea of higher order
> functions and lazy evaluation, and gain a lesson that comes
> much later, about the power that typeclasses add to composition.
> Come to think of it, you could use this to motivate typeclasses.
> I think you could build *just as good* an introductory Haskell
> course on the post-FTP libraries as you could on the pre-FTP
> libraries, but it would be a *different* course.

I think this is a good framing of the question
Lets say you take the subject matter for the introductory programming
And you topsort it along prerequisites; ie topic A precedes topic B if
understanding B needs knowledge of A
So is the structure/topography of the language Haskell conformant with this
topsort? Or does one need to jump against the ordering at times?

As example, take a course using C to teach programming. And consider input
vs pointers.
You have one of 3 choices:
1. Pointers before input -- you probably know programming earlier!
2. Input before pointers -- use getchar not scanf and laboriously write
atoi etc before anything else -- classic K&R
3. Bardur's solution -- "just ignore that bit (the '&') , I'll explain when
you're ready" For a one-off case that's ok; when it happens at every turn
teaching/learning becomes a nightmare
The C version of that is described in this old paper: C in education and
software engineering

I just hope Haskell does not repeat that history -- especially considering
that this whole discussion starts with the need to distinguish
pointer-types and non-pointer types
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